Veneer is a layer of wood of superior value or excellent grain for overlaying an inferior wood. Veneer is usually formed from a thin sheet of wood that is cut or sawed from a log and is adapted for adherence to a smooth surface. Sheets of veneer are extremely thin (usually 0.5 to 0.7 mm in thickness). The length of the veneer sheets is typically in the range of 2.2-5.2 m. The width of the sheet of veneer can range from 200 to 1000 mm. These "raw" sheets of veneer contain moisture which must be removed by a drying process prior to lacquering or other laminating processes. The drying process normally includes applying heat and pressure to the veneer inside a temperature and humidity controlled dryer as is known to persons of ordinary skill in the art.
Moving the sheets of veneer through the dryer during the drying process is a delicate operation because the wet veneer sheets lack sufficient weight or rigidity to be moved easily in conveying systems. Also, the veneer shrinks during the drying process. If the sheet is being held firmly while it is trying to shrink, the resulting stresses can cause cracking or microfissures. Microfissures cause the surface porosity of the veneers to vary, thus creating problems in the subsequent laminating operations.
There are two principal methods for conveying sheets of veneer through dryers. The first method involves roller-type systems that pass the veneer sheets through successive pairs of opposed pressure rollers. The roller-type systems convey the sheets of veneer along the drying path and have a pressing effect that flattens and smooths out the surface of the veneer sheets. However, these systems have some drawbacks, including the fact that the elevated pressures associated with these type systems are often unsuitable for the treatment of some woods.
The disadvantages are especially serious for sensitive and highly valuable thin veneers. Veneer requires a certain amount of rigidity to be fed from roller pair to roller pair. This rigidity is not available in most thin or moist veneer. Accordingly, the roller systems have to have positive traction on the veneer by means of the rollers at all times. This traction does not adequately accommodate shrinking.
The second type of conveying system for thin veneers is the overlaid belt type systems. In these systems, a pair of endless belts are driven by guide rollers. The sheet of veneer is carried between the belts. The belts are usually manufactured out of a mesh material to allow the drying air inside the dryer to reach the surface of the material.
Some of the belt type systems convey the sheets of veneer through the dryer in a simple rectilinear path. Although this type of belt arrangement allows the sheets to shrink during drying, they typically do not provide a sufficient pressing effect. According to the particular form of the belts (for example wire netting), a smoothing effect caused by the weight of the upper or covering belt may occur. However, this smoothing effect is not sufficient in prior art belt drying processes to prevent the appearance of wavelike imperfections on the product during drying. Accordingly, the main advantages to simple rectilinear belt dryers are that it is possible to shrink the veneer between the belts to avoid crack formation and that the systems are capable of feeding thin, moist veneer. However, the weight of the covering belt is generally not sufficient to smooth the veneer satisfactorily. Accordingly, the veneer will usually have to be dried in belt dryers and then smoothed in a smoothing press in order to remove the imperfections.
In order to provide better flattening of the sheets, a drying system having a belt conveying system arranged in a sinusoidal path is described and illustrated in German patent No. 1266233. The belts are guided on a curved, looped path around staggered rollers. The sheet to be dried is entrained between two endless smooth wire fabric belts, which are passed zig-zag-wise in a controllable arrangement about the rollers with an elevated belt tension. As a result of the sinusoidal path, the belts are able to bring a flattening pressure onto the veneers. However, such pressure is continuous throughout the drying cycle and does not adequately compensate for shrinking. In order to maintain the pressure, the sheets are required at almost every instant to follow a curvilinear path during which they are practically always in contact with the guide rollers. As a result, the veneers are not permitted to shrink freely without risk of causing flaws or cracks.
U.S. Pat. No. 4,654,981 discloses an improvement in the opposed belt, sinusoidal drying path system wherein between one guide roller and the next (the axes of rotation of which are all in one plane) there is a rectilinear section having a length equal to at least one-half of the maximum width of the veneer sheets. Although the rectilinear sections provide for shrinking, the curvilinear sections created by the large guide rollers create pressures on the veneer sheets that are too great for certain very thin, superior wood veneers and can cause microfissures. Also, the arrangement of the guide rollers in the same plane presents a problem with mounting nozzles for blowing warm air directly onto the veneers contained inside the belts. Accordingly, the drying process has to be lengthy or the rollers have to be equipped with costly heating elements.
Another improvement over the opposed belt sinusoidal drying path system is disclosed in U.S. Pat. No. 4,862,600. This patent discloses a drying path wherein the guide rollers are smaller and are positioned in a sinusoidal path instead of being all in the same plane. A plurality of rectilinear sections are disposed between the guide rollers, and a curvilinear section is disposed along each roller. However, due to the orientation of the guide rollers the belts only wrap around the primary roller in the curvilinear section for an angle of approximately 40 to 50 degrees of rotation. The problem with conveying the sheets of veneer in this manner is that it would be exceedingly difficult to stretch a metallic belt under the required tension around so many small rolls. The rolls would not be able to withstand the load.
What is needed is a drying system with a drying path that does not cause cracking or microfissures on the surface of the veneer, that provides for pressing of the veneer to remove surface imperfections, and that is suitable for drying and conveying very thin, delicate and lightweight veneers.